The Somatosensory System Structure and function L. Négyessy PPKE, 2011
Somatosensation Touch Proprioception Pain Temperature Visceral functions
I. The skin as a receptor organ Sinus hair Merkel endings are also found in the sinus hair (2,5). Note the clustering and annular distribution of receptors (2,5,6) in the sinus follicle. 3
Touch receptors Meissner Merkel Paccini Ruffini Free nerve ending ridges: 0.5 mm, grooves: 0.25 mm 4
The Merkel cell - structure Cross section of the skin. Arrows indicate the localization of Merkel cells deep in the epidermis at the border with the dermis. Scanning electron micrograph. Cell surface processes: diam.: 0,1-0,25 mm length: ~1 mm (<2,5 mm) 5
The Merkel cell - function Receptor potential genesis St: stimulus RP: response Me: Merkel cell; Ne: nerve ending 6
Mechanosensitive channels 7
Modeling the skin I: Continuum Mechanical Model physical quantities closely related to local membrane stretch were most predictive of the observed afferent responses. 8
Modeling the skin II: 3D Finite Element Model the elastic behavior of skin is nonlinear and can be divided into three regions: an initial region of low elastic modulus, a transition region, and a final region with high elasticity 9
Exploring the skin: Magnetic resonance elastography (MRE) 10
II. Major somatosensory pathways: Lemniscus medialis & Tractus spinothalamicus Touch, proproception Pain, temperature 11
II.a. Topographic organization Labeled lines 12
The vibrissal somatosensory pathway of rodents 13
Spinal cord The spinal rootlets, contributing to one nerve, arise from one spinal segment. Each segment is a functional unit, related to a region of the body. Limited independence - controlled by the CNS (brain stem and cortex) - descending tracts Intersegmental coordination - ascending fibres to higher centres - propriospinal fibres within the cord 14
The dermatomes 15
II.b. Functional representations Thalamocortical loop 16
The cortical somatosensory map 17
The hand representation area 18
Somatotopy and the funneling ilusion A merging index (MI) was designed to measure the spatial shift in the activation spot location. The merging index (MI) ranges from 1 (cortical location of one digit) to 0 (centre between two digits) to 1 (cortical location of other digit). Under two digit stimulation conditions, the center of digit activation can shift either towards the center (MI < 1 ) or away from the center (MI > 1 ). 19
Functional vibrotactile maps: submodalities 20
Sub-barrel column direction map 21
Further cortical processing 22
III: Receptive field (RF) organization 23
Subcortical origin of surround RF 24
RF characteristics 25
Orientation and direction sensitivity 26
Spatio-temporal dynamics of RF Lagged inhibition: 30 ms delay 27
The 3 component RF model Fix components: -orientation selectivity -spatial filter (selectivity for spatial features, patterns) Lagged inhibition: -stimulus gradient selectivity -direction selectivity Note the fixed relative position of the excitatoiry and fixed inhibitory components. Only the lagged inhibitory component changes its position. 28
Comparison of Peripheral and cortical RFs 29
Summary of RF organization The three-component RFs predicted orientation sensitivity and preferred orientation to a scanned bar accurately. The orientation sensitivity was determined most strongly by the intensity of the coincident RF inhibition in relation to the excitation. The fixed excitatory and inhibitory components of each neuron function as a spatial filter, conferring selectivity for particular spatial features or patterns regardless of scanning direction and velocity. The lagged inhibitory component confers selectivity for stimulus gradients in the scanning direction, regardless of that direction. To the extent that its lag center is displaced from the center of excitation, it also functions, at least theoretically, as a basis for directional sensitivity. 30
IV. Encoding stimulus attributes in SI: Texture discrimination 31
Velocity invariance 32
Vibrotactile discrimination 33
Population response 34
SUMMARY Vibrotactile receptors Somatosensory pathways, labeled lines Vibrotactile cortical maps RF organization (3 component model) RF characteristics: selectivity for stimulus features Neural correlates of texture and vibrotactile discrimination Basics of population coding 35